1. Field of the Invention
The present invention relates to adaptive predistorters for compensating time varying nonlinear amplitude and phase distortion of high power amplifiers used for transmission of data in digital communication systems.
2. Description of the Prior Art
Digital transmission application, such as wireless and wireline video, multimedia, audio and data transmission, have created the need for high data throughput in bandlimited channels. To obtain the required throughput, multi-amplitude and multi-phase modulation schemes such as Quadrature Amplitude Modulation (QAM), Vestigial Sideband Modulation (VSB), and Coded Orthogonal Frequency Division Multiplexing (COFDM) are being utilized. These modulation schemes increase the order of modulation thereby providing the required throughput. However, increasing the order of the modulation also increases the sensitivity of these modulation schemes to distortion. Typical sources of distortion include, for example, High Power Amplifier (HPA) nonlinearity, multipath distortion, additive noise, and analog circuit imperfection. Moreover, digital transmission applications, such as, digital television, are subject to stringent out-of-band emission requirements. The sensitivity to distortion and the stringent out-of-band restrictions require that the output of HPAs not be contaminated with nonlinear amplification products.
In HPAs, these nonlinear amplification products include nonlinear amplitude-dependent phase distortion (AM/PM) and nonlinear amplitude-dependent amplitude distortion (AM/AM). The AM/PM and AM/AM distortions are produced by the transistors of the HPA operating in nonlinear regions of their operation. For example, very large input signals may cause the transistors to saturate and very small input signals may have insufficient amplitude to cause the transistors to conduct.
One approach to eliminate this distortion includes limiting the amplitude of the input signal so that the HPA operates within its linear operating range. This approach, however, fails to utilize the full amplification capability of the HPA. Another approach is to predistort the input signal so that the HPA produces a desired, undistorted output.
Signal predistortion typically includes pre-amplifying and pre-phase shifting the input of the HPA to compensate for the saturation and phase shift thereof. One method of predistortion includes mapping predistorted data, comprised of an in-phase portion and a quadrature portion of a complex data signal, into a lookup table. In response to the input of a complex signal from a remote source, predistorted data are selectively retrieved from the lookup table, modulated and passed through the HPA. If the predistorted data are properly selected, the demodulated output of the HPA will be a linearly amplified version of the input complex signal from the remote source. However, the manner in which the inverse of the HPA nonlinearity is estimated and implemented affects the creation of the lookup table.
One problem with prior art predistortion methods is that input signals undergoing the most severe distortion are the signals having the largest or smallest amplitude. Moreover, signals having the largest or smallest amplitudes are also signal levels which are least likely to occur. Hence, prior art predistortion methods which utilize the output of the HPA to determine the best inverse to the output of the HPA have the least data where it is most needed. Another problem with prior art predistortion methods is that they do not take advantage of the correlation between distortion experienced by signals of similar power levels. Specifically, distortion created for one input signal amplitude is highly correlated with distortion created for a similar, but distinct, input signal amplitude. The lookup table values of prior art predistorters are continuously updated as a function of input signals being amplified immediately preceding update of the lookup tables. Hence, different input signals having similar input signal amplitudes may produce different lookup table values. These different lookup table values are caused by measurement distortion of the signal output by the HPA. Typical sources of measurement distortion include thermal noise, frequency response and group delay response of analog filters in the modulation and demodulation conversion process, phase noise of local oscillators utilized in the modulator and demodulator, and timing errors in the conversion of analog signals to digital signals.
Another problem with prior art predistortion methods is the absence of determining the gradient of the AM/AM distortion of the HPA. Not including the gradient of the AM/AM distortion of the HPA in the determination of the optimal inverse AM/AM HPA predistorted data increases the error in the AM/AM inverse estimate.